Thrust bearing component

ABSTRACT

A thrust bearing component is formed of cold-reduced steel sheets and strip having surface roughness of Rmax≦2 μm and provided by cold rolling a high-carbon steel containing 0.9 wt % to 1.2 wt % of carbon, 1.2 wt % to 1.7 wt % of chrome, 0.1 wt % to 0.5 wt % of manganese, and 0.15 wt % to 0.35 wt % of silicon.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thrust bearing component such as abearing washer of a thrust rolling bearing or a thrust sliding bearing,and a bearing washer of a thrust receiving mechanism.

2. Description of the Background Art

A high mechanical property and high processing accuracy are required fora bearing washer of a thrust rolling bearing or a thrust slidingbearing, and a bearing washer of a thrust receiving mechanism (referredto as the “thrust bearing component” collectively hereinafter).Therefore, when the thrust bearing component is manufactured, it isessential to select an appropriate starting material and an optimalprocessing method appropriate for the material.

For example, a manufacturing method of a synchronizer ring whosestructure is similar to that of the thrust bearing component isdisclosed in Japanese Unexamined Patent Publication No. 11-223225. Themanufacturing method of the synchronizer ring disclosed in the abovedocument will be described with reference to FIG. 6.

First, the synchronizer ring disclosed in the above document uses asteel plate provided by hot rolling a carbon steel containing 0.6 wt %to 1.2 wt % of carbon (C), 0.1 wt % to 0.9 wt % of manganese (Mn), 0.3wt % to 1.0 wt % of chrome (Cr), and 0.01 wt % to 0.15 wt % of silicon,as a starting material.

Thus, it is disclosed that the synchronizer ring is manufactured througha step of forming the above starting material into a predetermined shapeby a pressing process, a step of providing a predetermined dimension bya machining process such as a turning process, a step of providingpredetermined hardness by a heat treatment including quenching andtempering, and a step of smoothing the surface by a grinding process asa finishing process.

Recently, the environment in which the thrust bearing is used has becomeincreasingly harsh, and accordingly it is required that the mechanicalproperty of the thrust bearing washer is to be further improved.Meanwhile, as price reduction of the thrust bearing is required, themanufacturing process needs to be simplified without changing itsquality.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a thrust bearingcomponent whose manufacturing cost is lowered by simplifying amanufacturing process and mechanical property is further improved.

A thrust bearing component according to the present invention is formedof a cold-reduced steel sheets and strip having surface roughness ofRmax≦2 μm and provided by cold rolling a high-carbon steel containing0.9 wt % to 1.2 wt % of carbon (C), 1.2 wt % to 1.7 wt % of chrome (Cr),0.1 wt % to 0.5 wt % of manganese (Mn), and 0.15 wt % to 0.35 wt % ofsilicon (Si).

Preferably, the thrust bearing component is manufactured by a step offorming the cold-reduced steel sheets and strip into a predeterminedshape by a pressing process, a step of performing a heat treatmentincluding a carbonitriding process and a high-temperature temperingprocess, and a step of removing a scale generated on its surface.

When the carbon steel having the above chemical components is used, themechanical property of the thrust bearing component is improved. Morespecifically, a quenching property is improved, a rolling fatigue lifeand a load bearing property are improved, friction and wear are reduced,hardness is improved, and damage of the thrust bearing component due tothe pressing process and the like can be prevented.

In addition, since the steel plate manufactured through the cold rollingprocess can provide desired dimension, surface smoothness, and hardness,a turning process for adjusting the dimension and a grinding process forsmoothing the surface and the like can be omitted in the manufacturingprocess of the thrust bearing component. Thus, since the manufacturingprocess of the thrust bearing component can be simplified, themanufacturing cost of the thrust bearing component can be lowered.

According to the present invention, since the steel plate for the thrustbearing component provided by cold rolling the carbon steel having thepredetermined chemical components is used as a starting material, thethrust bearing component can be low in cost and superior in mechanicalproperty.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a thrust needle roller bearing according to oneembodiment of the present invention;

FIG. 2 is a flowchart showing main steps of the manufacturing process ofa steel plate for a thrust bearing component;

FIG. 3 is a flowchart showing main steps of the manufacturing process ofa thrust bearing component from the steel plate for the thrust bearingcomponent;

FIG. 4 is a view showing a thrust sliding bearing according to anotherembodiment of the present invention;

FIG. 5 is a view showing a thrust sliding bearing according to stillanother embodiment of the present invention: and

FIG. 6 is a flowchart showing main steps of the manufacturing process ofa synchronizer ring.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A thrust needle roller bearing 11 and a manufacturing method of bearingwashers 12 and 13 of the thrust needle roller bearing 11 according toone embodiment of the present invention will be described with referenceto FIGS. 1 to 3. In addition, FIG. 1 is a view showing the thrust needleroller bearing 11, FIG. 2 is a flowchart showing main manufacturingsteps of a cold-reduced steel sheets and strip that is a startingmaterial of a thrust bearing component, and FIG. 3 is a flowchartshowing main manufacturing steps of the thrust bearing component.

First, referring to FIG. 1, the thrust needle roller bearing 11comprises a pair of upper and lower bearing washers 12 and 13, aplurality of needle rollers 14 arranged between the pair of bearingwashers 12 and 13 radially, and a retainer 15 retaining the interval ofthe adjacent needle rollers 14.

According to the above thrust needle roller bearing 11, although thereare various advantages such that load capacity and rigidity can be highwith a simple form, differential sliding is generated between thebearing washers 12 and 13 and the needle roller 14. According to theneedle roller 14, pure rolling is provided in the center in itslongitudinal direction and relative sliding is linearly increased towardboth ends thereof. Especially, since the needle roller bearing 14 islong, the difference in peripheral speed at both ends of the needleroller 14 is large and a sliding amount is large as compared with otherbearings.

Thus, since an wear amount of the bearing washers 12 and 13 is large ata part in which the differential sliding is largely generated, surfaceorigin type peeling is generated at an rolling track edge. Especially,since the thrust needle roller bearing 11 have many rollers and narrowin internal space, lubricant oil is not likely to be spread on the tracksurface. As a result, the surface origin type peeling is easilygenerated as compared with the other bearings.

In addition, a high thrust load is applied to the bearing washers 12 and13 in the thrust needle roller bearing 11 having the above constitution.Furthermore, predetermined hardness and surface smoothness are requiredfor the track surface on which the needle roller 14 rolls.

Thus, a description will be made of a manufacturing method of the steelplate for the thrust bearing component that will be the startingmaterial of the thrust bearing component to be used in the aboveenvironments with reference to FIG. 2. First, a billet containing 0.9 wt% to 1.2 wt % of carbon (C), 1.2 wt % to 1.7 wt % of chrome (Cr), 0.1 wt% to 0.5 wt % of manganese (Mn), 0.15 wt % to 0.35 wt % of silicon (Si),the remainder of inevitable impurities, and iron (Fe) is prepared as thestarting material (S11). In addition, it is to be noted that the oxygenconcentration in steel is not more than 0.0010%.

Carbon (C) is an essential element to ensure the strength required forthe thrust bearing component. In addition, 0.9 wt % of carbon is neededto ensure the hardness of HRC58 or more on the surface and core part ofthe thrust bearing component. Meanwhile, when the carbon content exceeds1.2 wt %, a large carbide is generated on the surface of the thrustbearing component, which lowers the rolling fatigue life and loadbearing property and increases friction and wear. Thus, it is desirablethe carbon content is within a range of 0.9 wt % to 1.2 wt %. Inaddition, the “HRC” designates Rockwell hardness.

In addition, chrome (Cr) is an essential element to improve thequenching property and rolling fatigue life of the thrust bearingcomponent, ensure the hardness by the carbide, prevent the friction andwear, and improve the load bearing property. In addition, 1.2 wt % ormore of chrome is needed to obtain the predetermined carbide. Meanwhile,even when an amount exceeding 1.7 wt % is added, its added effect is notprovided so much. When it exceeds 5.0 wt %, a large carbide isgenerated, which lowers the rolling fatigue life and load bearingproperty and increases the friction and wear. Thus, it is desirable thechrome content is within a range of 1.2 wt % to 1.7 wt %.

In addition, manganese (Mn) is an element used for deoxidation when thesteel is manufactured and it is an essential element as the startingmaterial of the thrust bearing component. In addition, 0.1 wt % ofmanganese is needed to remove oxygen from the steel satisfactorily.Meanwhile, when more than 0.5 wt % of manganese is added, the materialbecomes fragile and the thrust bearing component could be damaged at thetime of pressing. Thus, it is desirable that the manganese content iswithin a range of 0.1 wt % to 0.5 wt %.

In addition, silicon (Si) is an essential element in the steel materialand the lower limit value of its content is 0.15%. Meanwhile, when itexceeds 0.35 wt %, the thrust bearing component could be damaged at thetime of pressing. Thus, it is desirable that the silicon content iswithin a range of 0.15 wt % to 0.35 wt %.

In addition, since oxygen forms an oxide in steel, that becomes anorigin of fatigue destruction as a nonmetal inclusion, it reduces therolling fatigue life and load bearing property and increases thefriction and wear. Thus, it is desirable that the oxygen concentrationin steel is not more than 0.0010%.

Then, the steel plate is formed of the above material by hot rolling(S12). A large cast structure becomes a fine and preferable rollingtexture through the hot rolling. In addition, since the work hardeningof the material can be prevented by rolling at a temperature more than arecrystallization temperature, the thickness can be immediately reduced.

In addition, a step of annealing the rolled steel plate may be addedafter the hot pressing step. Since a crystal grain becomes fine and thedirection of the crystal is adjusted by annealing, the accuracy andoperability of the surface can be improved.

Then, acid pickling is performed in order to prevent rust and removeoxide layer (scale) attached on the surface of the steel plate (S13).The oxide layer shortens the life of a tool in a machining process andlowers production efficiency, and causes physical and chemical changeson the surface of the steel plate to lower the effect of the surfaceprocessing. Thus, as the oxide layer is removed through the acidpickling, production efficiency in the following steps and productquality can be improved. In addition, a pickling solution includeshydrochloric acid, sulfuric acid, nitric acid and the like, and 5% to15% of dilute hydrochloric acid solution is used at 40° C. to 50° C. inmany cases.

Then, the steel plate reaches a predetermined size and machineryproperties such as hardness and surface smoothness and the like requiredfor the thrust bearing component are provided by cool rolling (S14). Apredetermined plate thickness can be accurately provided and uniformsmoothness can be provided by rolling at a room temperature. Inaddition, since the steel plate is work-hardened by rolling at atemperature below the recrystallization temperature, the hardness of thesteel plate is improved.

In addition, the surface roughness of the wall surface to become thetrack surface of the thrust bearing component is required to be Rmax≦1.6μm in view of the smooth rolling of the needle roller 14. As will bedescribed below, since barreling can be only preformed as far as a headof the surface roughness is removed after the shape of the thrustbearing component has been processed, it is desirable that the surfaceroughness after the cold rolling process is such that Rmax≦2 μm.Furthermore, in view of preventing the damage at the time of pressforming, the hardness after the cold rolling process is preferably Hv220or less. Here, the “Rmax” designates a maximum height and “Hv”designates Vickers hardness.

Here, the surface roughness, hardness and thickness of the steel plateprovided by the cold rolling process are affected by the surfaceroughness of the rolling roll, the flexibility of the rolling roll, arolling ratio (ratio of the thickness between after and before therolling), the gap between the rolling rolls, a rotation speed and thelike. Therefore, in order to provide the desirable surface roughness,hardness and plate thickness, it is necessary to set the above elementsappropriately.

In addition, although the predetermined thicknesses may be provided bythe above hot rolling process and the cold rolling process respectively,the predetermined thickness may be provided through a plurality ofprocesses such as rough rolling, middle rolling and finish rolling.

When the billet comprising the above chemical components is processed bythe above processes, the steel plate for the thrust bearing component(cold-reduced special steel sheets and strip) can provide very highmechanical property. Thus, in order to confirm the effect of the presentinvention, tensile strength, 0.2% proof stress, total elongation atrupture, work hardening coefficient (n value) and plastic strain ratio(r value) of the above steel plate for the thrust bearing component weremeasured. The measured result is shown in Table 1. In addition, in Table1, a minimum value, a maximum value and average value are shown withrespect to each lot (No. 1 to No. 3).

TABLE 1 Total elongation at Work hardening Tensile strength 0.2% proofstress rupture coefficient Plastic strain ratio Mini- Mini- Mini- Mini-Aver- Mini- Aver- mum Maximum Average mum Maximum Average mum MaximumAverage mum Maximum age mum Maximum age value value value value valuevalue value value value value value value value value value 560 622 589388 458 428 28 32 30 0.15 0.16 0.16 0.96 1.48 1.26 572 628 596 364 452406 27 31 29 0.15 0.16 0.16 0.94 1.46 1.22 586 638 607 394 462 434 26 3028 0.15 0.16 0.15 0.92 1.52 1.21

Referring to Table 1, the tensile strength of the above steel plate was550N/mm² or more. In addition, as the tensile strength and the 0.2%proof stress become high, the total elongation at rupture and the workhardening coefficient and the plastic strain ratio become low as will bedescribed below. Since these values largely affect the workability ofthe steel plate, the material showing high values of those is notpreferable for the material for manufacturing the thrust bearingcomponent. Thus, although the maximum value of the tensile strength is638/mm², and the maximum value of the 0.2% proof stress is 462N/mm² inTable 1, it is preferable the tensile strength is preferablyapproximately 550N/mm² and the 0.2% proof stress is preferablyapproximately 350N/mm².

In addition, the total elongation at rupture of the above steel platewas 25% to 35%. In addition, the work hardening coefficient was 0.12 to0.18. Furthermore, the plastic strain ratio was 1.20 to 1.30. Inaddition, as the values of the total elongation at rupture, the workhardening coefficient and the plastic strain ratio are increased, theforming limit is improved, which is advantageous in manufacturing thethrust bearing component. However, according to the present invention,the values are set within the range described above in view of therelation with the tensile strength and the 0.2% proof stress.

In addition, the tensile strength, the 0.2% proof stress, and the totalelongation at rupture were measured based on JIS (Japanese IndustrialStandards) Z 2241. Similarly, the work hardening coefficient (n value)and the plastic strain ratio (r value) were measured based on JIS Z 2253and JIS Z 2254, respectively.

Next, a description will be made of the manufacturing method of thethrust bearing component according to one embodiment of the presentinvention with reference to FIG. 3. In addition, FIG. 3 is a flowchartshowing main manufacturing steps of the thrust bearing component. First,the steel plate (cold-reduced steel sheets and strip) for the thrustbearing component described with reference to FIG. 2 is employed as thestarting material (S21).

Then, the steel plate is formed into the thrust bearing component bypressing (S22). According to the above starting material, since itsthickness, surface roughness and the like are already in preferableconditions by the cold rolling process, the process such as a turningprocess may be omitted. As a result, since the manufacturing process canbe simplified, the manufacturing cost of the thrust bearing componentcan be low. In addition, although the preferable configuration may beprovided by the one pressing process, the preferable configuration maybe provided through the plurality of pressing processes. In addition, aburring process may be performed after the pressing process.

Then, in order to provide the mechanical property required for thethrust bearing component, a heat treatment including a carbonitridingprocess and high-temperature tempering is performed (S23). In addition,a nitrogen enrichment layer is formed on the surface of the thrustbearing component by the carbonitriding process. This nitrogenenrichment layer is effective in improving the rolling fatigue life andload bearing property and in reducing the friction and wear.

In addition, not only the high temperature resistance characteristicsare improved but also the residual austenite is split into temperedmartensite and fine carbide by the high temperature tempering, which iseffective in improving the rolling fatigue life and the load bearingproperty and in reducing the friction and wear under a high loadcondition, especially.

In addition, a tempering temperature needs to be 230° C. or more to keepthe residual austenite below 5%. Meanwhile, when the temperingtemperature is 280° C. or more, the hardness becomes HRC60 or less,which could not maintain the hardness required for the thrust bearingcomponent. Thus, it is preferable that the tempering temperature iswithin a range of 230° C. to 280° C.

Finally, the oxide layer (scale) generated on the surface of the thrustbearing component during the heat treatment is removed (S24). The scaleremoving process includes a mechanical method such as a barrelingprocess and blast cleaning or a chemical method such as acid pickling asdescribed above.

Here, the “barreling process” is a process in which the thrust bearingcomponent, a compound and a medium are put into a barrel and the barrelis rotated or vibrated. By this method, the scale can be removed and thethrust bearing component can be burred and the surface roughness can beimproved. Since the surface roughness of the starting material of thethrust bearing component is already such that Rmax≦2 μm at the stageafter the cold rolling process as described above, the surface roughnesssuch that Rmax≦1.6 μm required for the thrust bearing component can beprovided without an independent grinding process.

According to the present invention, when the carbon steel having theabove chemical components is used, the various mechanical properties ofthe thrust bearing component are improved. As a result, the rollingfatigue life and load bearing property are improved and the friction andwear are reduced in the thrust bearing component.

In addition, when the cold rolling process is contained in themanufacturing process of the starting material (shown in FIG. 2), thethickness, hardness, surface roughness and the like required for thethrust bearing component can be provided. Thus, the turning process andthe grinding process can be omitted in the manufacturing steps of thethrust bearing component (shown in FIG. 3). As a result, themanufacturing process of the thrust bearing component is simplified andthe manufacturing cost of the thrust bearing component can be lowered.

In addition, although the description has been made of the manufacturingmethod of the bearing washers 12 and 13 of the thrust needle rollerbearing 11 in the above embodiment, the present invention is not limitedto this and the present invention can be applied to a manufacturingmethod of another thrust bearing component. For example, the presentinvention may be applied to a thrust rolling bearing in which a rollingelement is a cylindrical roller or a ball, or a thrust sliding bearinghaving no rolling element. Thrust bearings according to otherembodiments of the present invention will be described with reference toFIGS. 4 and 5. In each of FIGS. 4 and 5, an upper part is a perspectiveview and a lower part is a side sectional view.

First, referring to FIG. 4, a thrust sliding bearing 21 according toanother embodiment of the present invention comprises two bearingwashers 22 and 23. The bearing washers 22 and 23 are disk-shaped membershaving holes 22 a and 23 a in the center, respectively and they overlapwith each other so that their track surfaces 22 b and 23 b abut on eachother.

According to the thrust sliding bearing 21, the bearing washer 22 isfixed to a rotation shaft (not shown) and the bearing washer 23 is fixedto a housing (not shown), for example. Since the bearing washer 22rotates while slides on the bearing washer 23, the rotation shaft can berotatably supported. In addition, it is to be noted that the aboverotation shaft includes not only the one rotating in a certain directionbut also the one that oscillates. In addition, it is to be noted thatthe bearing washer 23 includes the one that is fixed to another rotationshaft relatively rotating with the above rotation shaft.

Next, referring to FIG. 5, a thrust bearing component 31 according tostill another embodiment of the present invention comprises two bearingwashers 32 and 33. The bearing washers 32 and 33 are rectangular membersand overlap with each other so that their track surfaces 32 a and 33 aabut on each other.

According to this thrust sliding bearing 31, the bearing washer 32 isfixed to a reciprocating member (not shown) and the bearing washer 33 isfixed to a housing (not shown), for example. Since the bearing washer 32reciprocates (shown by an arrow in FIG. 5) while slides on the bearingwasher 33, it can support the reciprocating member moving linearlywithin a certain range. In addition, it is to be noted that the bearingwasher 33 includes the one that is fixed to another reciprocating membermoving relatively with the above reciprocating member.

Large thrust load is applied to the bearing washers 22, 23, 32 and 33 inthe thrust sliding bearing 21 and 31, and certain hardness and surfacesmoothness are required for the track surfaces 22 b, 23 b, 32 b and 33a. Thus, when at least one of the bearing washers 22 and 23 of thethrust sliding bearing 21 and at least one of the bearing washers 32 and33 of the thrust sliding bearing 31 are manufactured by the method shownin FIGS. 2 and 3, the effect of the present invention can be provided.

In addition, it is to be noted that the “thrust bearing component” inthis specification includes not only the disk-shaped bearing washers 12,13, 22, and 23 as shown in FIGS. 1 and 4 but also the rectangle-shapedbearing washers 32 and 33 as shown in FIG. 5.

Although the embodiments of the present invention have been describedwith reference to the drawings in the above, the present invention isnot limited to the above-illustrated embodiments. Various kinds ofmodifications and variations may be added to the illustrated embodimentswithin the same or equal scope of the present invention.

The present invention can be advantageously applied to manufacturing ofthe bearing washer and the like of the thrust rolling bearing or thethrust sliding bearing.

1. A thrust bearing component formed of cold-reduced steel sheets andstrip having surface roughness of Rmax≦2 μm and provided by cold rollinga high-carbon steel containing 0.9 wt % to 1.2 wt % of carbon, 1.2 wt %to 1.7 wt % of chrome, 0.1 wt % to 0.5 wt % of manganese, and 0.15 wt %to 0.35 wt % of silicon.
 2. The thrust bearing component according toclaim 1, wherein said thrust bearing component is manufactured by: astep of forming said cold-reduced steel sheets and strip into apredetermined shape by a pressing process; a step of performing a heattreatment including a carbonitriding process and a high-temperaturetempering process; and a step of removing a scale generated on itssurface.